Configurable multi-lane scrambler for flexible protocol support

ABSTRACT

Various structures and methods are disclosed related to configurable scrambling circuitry. Embodiments can be configured to support one of a plurality of protocols. Some embodiments relate to a configurable multilane scrambler that can be adapted either to combine scrambling circuits across a plurality of lanes or to provide independent lane-based scramblers. Some embodiments are configurable to select a scrambler type. Some embodiments are configurable to adapt to one of a plurality of protocol-specific scrambling polynomials. Some embodiments relate to selecting between least significant bit (“LSB”) and most significant bit (“MSB”) ordering of data. In some embodiments, scrambler circuits in each lane are adapted to handle data that is more than one bit wide.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 12/847,761 filed on Jul. 30, 2010. The entire disclosure ofthat application is hereby incorporated by reference herein.

BACKGROUND

This invention relates generally to the area of system interconnecttechnology.

As CPU speeds have reached the multi-gigahertz range, system designersincreasingly focus on system interconnect as the primary bottleneck atthe chip-to-chip, board-to-board, backplane and box-to-box levels.System interconnect has evolved from utilizing parallel I/O technologywith source-synchronous clocking or system-synchronous clocking tomulti-gigabit serial I/O with clock-data recovery (“CDR”). Channelaggregation bonds individual serial I/O lanes to create a multi-lanelink, transcending the bandwidth limitations of single transceiverchannels and providing the high bandwidth required by next generationserial protocols such as 40/100 Gigabit Ethernet and PCI Express Gen 3.However, various communication protocols have different functionalrequirements. At the same time, there is an increasing need for systemdesigners to have flexibility in designing systems to work with oneparticular protocol versus another.

Scrambling/descrambling processing stages in a high speed transceiverallow high speed signals to have sufficient transition densities to helpminimize data errors. Scrambling is typically carried out by linearfeedback shift register (“LFSR”) circuits including shift registerelements and one or more XOR circuits. However, different protocols havedifferent scrambling techniques. For example, some protocols usemultiplicative scrambling while others use additive scrambling. Asanother example, different protocols use different scramblingpolynomials, each of which require different couplings to XOR circuitsin an LFSR. Also, in some protocols lanes are aggregated and data acrossmultiple lanes is preferably scrambled together. As another example,some protocols use a least significant bit (“LSB”) ordering of datawhile others use a most significant bit (“MSB”) ordering.

There is a need for integrated circuits (“ICs”) with transceivers thatcan be adapted for use with different protocols. However, it may becumbersome/costly to provide completely separate scrambling circuitryfor each possible protocol for which the IC might be utilized.Therefore, there is a need for scrambling circuitry that can be adaptedfor different protocols.

SUMMARY

An embodiment of the invention provides scrambling circuitryconfigurable to support one of a plurality of scrambling protocols. Inone embodiment, scrambling circuitry includes scrambling circuits ineach of a plurality of data lanes and the scrambling circuitry isconfigurable such that scrambling circuits in a plurality of data lanesare combined to provide a multilane scrambler or such that scramblingcircuits in each lane operate independently. In one embodiment, ascrambling circuit is configurable to be adapted for operation as one ofa plurality of scrambler types (for example, either additive ormultiplicative). In one embodiment, a scrambling circuit is configurableto support scrambling in accordance with any one of a plurality ofscrambling polynomials. In one embodiment, a scrambling circuit resultsfrom a method of streamlining the number of XOR circuits (and, in someembodiments, the number of programmable taps coupling shift registerelement outputs to the XOR circuits) needed to support one of aplurality of scrambling polynomials. In a particular embodiment, LSFRcircuits in each lane are adapted to scramble multi-bit wide data. Inone embodiment, selection circuitry coupled to a multi-lane scrambler isconfigurable to provide either least significant bit (“LSB”) or mostsignificant bit (“MSB”) data ordering from a first to a last lane of aplurality of lanes. In one embodiment, selection circuitry coupled to ascrambling circuit for a particular lane processing multi-bit wide datais configurable to provide either LSB or MSB data ordering within aparticular lane.

BRIEF DESCRIPTION OF THE DRAWINGS

For purposes of illustration only, several aspects of particularembodiments of the invention are described by reference to the followingfigures.

FIG. 1 illustrates a configurable multilane scrambler in accordance withan embodiment of the invention.

FIG. 2 illustrates further details of one of the configurable linearfeedback shift register (“LFSR”) circuits of the embodiment of FIG. 1.

FIG. 3 illustrates an alternative embodiment of the LFSR circuitillustrated in FIG. 2. This alternative embodiment illustrates an LFSRcircuit streamlined in accordance with an aspect of an embodiment of theinvention.

FIG. 4 illustrates a method for providing a streamlined scrambler thatcan support scrambling for any one of a plurality of differentscrambling polynomial functions. The illustrated method is in accordancewith an embodiment of one aspect of the invention.

FIG. 5 illustrates a configurable individual shift register element ofthe LFSR circuit illustrated FIG. 2.

FIG. 6 illustrates scrambling lane circuitry in accordance with analternative embodiment of the invention adapted for a 4-bit wide datalane.

FIG. 7 illustrates scrambling lane circuitry in accordance with analternative embodiment of the invention adapted for a 5-bit wide datalane.

FIG. 8 illustrates circuitry coupled to provide configurability betweenleast significant bit (“LSB”) and most significant bit (“MSB”) orderingin accordance with an aspect of an embodiment the invention.

FIG. 9 illustrates MSB/LSB configuration circuitry coupled to anindividual lane that is more than one bit wide in accordance with anaspect of an embodiment of the invention.

FIG. 10 illustrates an exemplary data processing system including atransceiver with a configurable scrambler in accordance with anembodiment of the invention.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the invention, and is provided in the context ofparticular applications and their requirements. Various modifications tothe exemplary embodiments will be readily apparent to those skilled inthe art, and the generic principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the invention. Thus, the invention is not intended to belimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the principles and features disclosed herein.

FIG. 1 illustrates a configurable multilane scrambler 100 in accordancewith an embodiment of the invention. Scrambler 100 includes linearfeedback shift register (“LFSR”) circuits 110, 120, 130, and 140associated with respective data lanes LANE 0, LANE 1, LANE 2, and LANE3. Scrambler 100 further includes a plurality of selection circuitscoupled between the LFSR circuits. The selection circuits illustrated inFIG. 1 include a plurality of output selection circuits includingmultiplexers (“muxes”) 102, 104, 106, and 108 and feedback muxes 101,103, 105, and 107. The terms “output” and “feedback” in the context ofthese selection circuits are merely labels to help identify theselection circuits in the context of this description; those terms arenot in and of themselves intended to impart any narrower meaning. Otherlabels are used herein for similar purposes, for example, simply to namedifferent outputs and inputs or label different circuits of the samebasic type (e.g. selection circuits, XOR circuits, etc.); as thoseskilled in the art will appreciate, such terms should in many instancesbe read merely as labels.

Scrambling circuits 110, 120, 130 and 140 have respective inputs forreceiving respective input data signals DATA_IN[0], DATA_IN[1],DATA_IN[2], and DATA_IN[3]. Scrambling circuits 110, 120, 130 and 140also each include inputs labeled “SCRM_IN” and outputs labeled“LFSR_OUT,” “DATA_OUT” and “SCRM_NEXT.” In some implementations, each ofthese signals, for example, DATA_IN[1], is one bit wide. However, inother implementations, the scrambling circuits may include circuitryadapted to receive parallel data in each lane that is more than one bitwide (sometimes referenced herein as “multi-bit” wide) as will be morefully described in the context of FIGS. 7-8.

Continuing with the description of FIG. 1, scrambler 100 can beconfigured such that scrambling circuits 110, 120, 130, and 140 eachscramble data independently on a lane-by-lane basis. For example, ifoutput muxes 102, 104, 106, and 108 are each configured to select their“A” inputs and feedback muxes 101, 103, 105, and 107 are each configuredto select their “B” inputs, then each of LFSR circuits 110, 120, 130,and 140 will be configured to operate independently and operate only ondata for a corresponding lane. However, one or more of the LFSR circuitsof scrambler 100 can be combined to work together such that data acrossmultiple lanes is scrambled together. For example, to combine (or“cascade”) the four illustrated LFSR circuits (110, 120, 130, and 140)to scramble data together across all four illustrated lanes, output mux108 would be configured to select its A input while output muxes 102,104, and 106 would be configured to select their B inputs. Also,feedback mux 101 would be configured to select its B input whilefeedback muxes 103, 105, and 107 would be configured to select their Ainputs. In this manner, output from LFSR circuits 120, 130, and 140 isprovided to the scrambling input SCRM_IN of LFSR circuit 110; alsooutput from SCRM_NEXT of LFSR 110 is provided to scramble input SCRM_INof LFSR 120, output from SCRM_NEXT of LFSR 120 is provided to scrambleinput SCRM_IN of LFSR 130, and output from SCRM_NEXT of LFSR 130 isprovided to scramble input SCRM_IN of LFSR 140.

FIG. 2 illustrates further details of an exemplary LFSR circuit, LFSRcircuit 120, of FIG. 1. LFSR 120 includes a series of shift registerelements 21, 22, 23, 24, 25, 26, 27, and 28. As those skilled in the artwill appreciate, these elements may be implemented by various well knowregister circuits such as, for example, flip-flops. The “D”, Q” “clock”and other inputs of these elements are well known but not separatelyshown to avoid complicating the drawings. However, those skilled in theart will appreciated that in a typical implementation, the data output“Q” of a first shift register element (e.g. element 24) will be coupledto a data input “D” of a next shift register element (e.g. element 25).Further details of the individual shift register elements and associatedselection circuitry are described in the context of FIG. 5. Continuingwith the description of FIG. 2, in this example, first inputs ofrespective AND gates 31, 32, 33, 34, 35, 36, and 37 are coupled torespective outputs of the shift register elements as shown. Secondinputs of respective AND gates are coupled, respectively, toconfiguration elements C1, C2, C3, C4, C5, C6, C7 and C8 as shown. Theoutputs of the AND gates are coupled to a feedback XOR circuit treeincluding feedback XOR circuits 212, 213, 214, 215, 216, and 217. Asillustrated, the output of XOR circuit 217 provides one of the inputs toinput XOR circuit 211 and is also coupled to the C input of mux 203 asshown. The AND gates together with the configuration elements effectiveprovide programmable taps from the shifter register elements to thefeedback XOR circuit tree. This allows the LFSR to be configured fordifferent polynomials (as will be further explained with specificexamples below). If the configuration element coupled to an AND gateinput is set to “1” (logical high), then the other input of the AND gatewill pass output from a shift register element to the XOR tree. On theother hand, if that configuration element is set to “0,” then thecorresponding shift register element output is not provided to the XORtree.

As illustrated in FIG. 2, the incoming data signal DATA_IN is providedto an input of XOR circuit 211, to the A input of mux 201, and to the Ainput of mux 203. Scrambling input signal “SCRM_IN” is provided to the Ainput of mux 202.

The selection circuits 201, 202, and 203 allow various configurationoptions for LFSR circuit 120. If mux 201 is configured to select its Ainput, then LFSR circuit 120 is effectively bypassed. This may bedesirable if, for example, the IC transceiver that includes the LFSRcircuit is to be configured for use with a protocol that does notrequire data scrambling. Furthermore, even if scrambling is utilized bya particular protocol, different protocols may require that thescrambling occur in a different order relative to other processing.Therefore, in a particular transceiver, two scrambling blocks may beprovided along a data path, but only one utilized depending upon whichprotocol is utilized. Therefore, mux 201 provides the ability for theparticular scrambling block of which LFSR 120 is a part to beselectively bypassed in the data path. Mux 202 is configured to selectits A input if the scrambler is in cascade mode (as described in thecontext of FIG. 1). However, assuming the scrambler is not operating incascade mode, then mux 202 is configured to select its B input. The Binput is coupled to receive the output of mux 203. Mux 203 allowsconfiguration of the “type” of scrambler circuit that LFSR circuit 120will be adapted to operate as. Specifically, when LFSR circuit 120 is tobe utilized as a multiplicative scrambler, then mux 203 is configured toselect its B input. When LFSR circuit 120 is to be utilized as amultiplicative descrambler, then mux 203 is configured to select its Ainput. When LFSR circuit 120 is to be utilized as an additive scrambleror descrambler, then mux 203 is configured to select its C input. Thoseskilled in the art will appreciated that in one embodiment selectioncircuits such as selection circuits 201, 202, and 203 can be configuredby loading data at power up into configuration elements (such as, forexample, random access memory elements) coupled to the control input ofthese multiplexers (such configuration elements not separately shown).However, the implementation of such configuration elements can varywidely. In some examples, fuse, anti-fuse, or other elements that are“one time” programmable may be used so that data does not have to beloaded into the element each time the device is powered up. The type ofprogrammable element utilized may vary by application as will beappreciated one skilled in the art. In some alternative examples, thecontrol of such muxes may be accomplished by signals provided “on thefly” by on chip processing logic (e.g. a state machine) that providesthe necessary control signal during operation of the IC of which thescrambler is a part. Such an “on the fly” control signal implementationmight be appropriate in some instance in which the value of the controlsignal might need to change after device initialization.

Different protocols utilize different scrambling polynomials. Differentpolynomials require different shift register element outputs to betapped to feed back into one or more XOR circuits. By programmingconfiguration elements C1, C2, C3, C4, C5, C6, C7, and C8, variousscrambling polynomials may be supported. As a simplified example, if thescrambling polynomial is of the form x⁵+x³+1, then C1, C2, C4, C6, C7,and C8 are programmed with “0” (logical low) and C3 and C5 areprogrammed with “1” (logical high). The effect of such a configurationis that the outputs shift register elements 23 and 25 are XOR'edtogether by XOR circuit 217.

As another simplified example, if the scrambling polynomial is of theform x⁸+x⁶+x²+1, then C2, C6, and C8 are configured with a 1 and C1, C3,C4, C5, and C7 are configured with a 0. The effect of such aconfiguration is that the outputs of shift register elements 26 and 28are XOR'ed together by feedback XOR circuit 216 and the result is XOR'edtogether with the output of shift register element 22 by feedback XORcircuit 217.

Those skilled in the art will appreciate that in actual implementations,scrambling polynomials often utilized terms with much larger exponentsthat the simplified examples referenced above. An LFSR circuit thatimplements such scrambling function would be larger than LFSR circuit120 illustrated in FIG. 2. The illustration of FIG. 2 is merely forpurposes of illustrating the underlying principles of an embodiment ofthe invention. As examples of some presently defined high speedcommunication protocols: The SONET/SDH protocol utilizes the polynomialx⁴³+1. The Ethernet/Interlaken protocol utilizes the polynomialx⁵⁸+x³⁹+1. The PCI Express Gen. 3.0 protocol uses the polynomialx²³+x²¹+x¹⁸+x¹⁵+x⁷++x²+1. Therefore, LFSR circuits need to be ofsignificantly greater length than that illustrated in FIG. 2. Forexample, to support any one of the three above-mentioned protocols, thenumber of shift register elements needed in a multi-protocol LFSRcircuit is given by the highest exponent of any term in any of thepossible scrambling polynomials that need to potentially be supported.In this case, 58 shift register elements would be needed. If all thepossible exponent values up to 58 were covered by the feedback XORcircuit tree tapping off from the shift register elements, the XORcircuit tree would be very large. However, given that protocols which aconfigurable IC transceiver would likely need to support are limited andnot likely to have every exponent value between the smallest and largestexponent across the various protocol-specific polynomials, it ispossible and desirable to provide a streamlined arrangement of feedbackXOR circuits that minimizes the circuitry needed to support the variousprotocols. A simplified example is illustrated in FIG. 3.

FIG. 3 illustrates an LFSR circuit 120′ that is a streamlined version ofLFSR circuit 120 illustrated in FIG. 2. The example of FIG. 3 assumesthat the supported protocols only have polynomial terms with exponentsequaling 1, 3, 4, 6 , or 8. Therefore, the full arrangement of XORcircuits illustrated in FIG. 2 is not necessary. As illustrated in FIG.3, only register elements 21, 23, 24, 26, and 28 need to be coupled tofeedback XOR circuitry through a respective AND gate. Furthermore, onlyfour feedback XOR circuits, circuits 212, 213, 216, and 217 are requiredin a XOR circuit tree (for purposes of simplifying the description,these XOR circuits, which XOR together outputs of shift registerelements, are referenced as part of a feedback “XOR tree” while inputXOR circuit 211, which XORs together the result of the feedback XOR treewith input data is referenced separately). The illustrated arrangementcan supported a plurality of scrambling polynomials. For example, tosupport a polynomial of the form x⁸+x⁴+x+1, configuration bits C1, C4,and C8 would be set to 1 and C3 and C6 would be set to 0. The effect ofsuch a configuration is that the outputs of shift register elements 21and 24 are XOR'ed together by XOR circuit 212 and the result is XOR'edtogether with the output of shift register element 28 by XOR circuit217. As another example to support a polynomial of the form x⁶+x³+1,configuration bits C3 and C6 would be set to 1 and C1, C4, and C8 wouldbe set to zero. The effect of such a configuration is that the outputsof shift register elements 23 and 26 are XOR'ed together by XOR circuit217.

FIG. 4 illustrates a method 400 for providing a streamlined scramblerthat can support scrambling for any one of a plurality of differentscrambling polynomial functions. Step 401 determines the largestexponent of a term across any of the plurality of polynomials. Step 402provides a number of shift register elements that is equal to the resultof step 401. For example, if the highest exponent of any term is 58,then 58 shift register elements would be provided. Step 403 identifiesthe value of non-zero exponents for each term in the supportedpolynomials. Step 404 provides programmable taps coupled to the outputof each shift register element in the chain of shift register elementsthat corresponds to the result of step 403. For example, if variousterms of supported polynomials have exponents including 5, 21, 30, and58, then the 5^(th), 21^(st), 30^(th), and 58^(th), shift registerelements in the shift register would have their outputs configurablycoupled (e.g. via programmable taps) to a XOR circuit tree. Under step405, the number of XOR circuits provided in the XOR circuit tree is Nwhere N+1 is the number of shift register elements with programmabletaps as determined in step 403. In the example of the 5^(th), 21^(st),30^(th), and 58^(th) shift register elements having programmable taps tothe XOR circuit tree and the XOR circuit tree would include four XORcircuits.

As stated above, the SONET/SDH protocol utilizes the polynomial x⁴³+1;the Ethernet/Interlaken protocol utilizes the polynomial x⁵⁸+x³⁹+1; thePCI Express Gen. 3.0 protocol uses the polynomialx²³+x²¹+x¹⁸+x¹⁵+x⁷+x²+1. To build a configurable scrambler to supportthese protocols applying method 400 (and following the principles of theexample of FIG. 3), an LFSR with 58 shift register elements would beprovided. Programmable AND gate taps would be provided coupling theoutputs of the 2^(nd), 7^(th), 15^(th), 18^(th), 21^(st), 23^(rd),39^(th), 43^(rd), and 58^(th) shift register elements to a XOR circuittree. The feedback XOR circuit tree would have eight XOR circuits andwould be coupled to an input XOR circuit. To configure such a circuit tosupport one of these protocols, one would follow the principles of theexample described in the context of FIGS. 2 and 3. Specifically, for theSONET/SDH protocol, the programmable AND gate coupled to the output ofthe 43^(rd) shift register element would be set to “1” and the remainingAND gate programmable inputs would be set to “0.” For the EthernetInterlaken Protocol, the programmable AND gates coupled to the outputsof, respectively, the 39th^(rd) and 58^(th) shift register elementswould be set to “1” and the remaining AND gate programmable inputs wouldbe set to “0.” For the PCI Express Gen. 3 protocol, the programmable ANDgates coupled to the outputs of, respectively, the 2^(nd), 7^(th),15^(th), 18^(th), 21^(st), and 23^(rd) shift register elements would beset to “1” and the remaining AND gate programmable inputs would be setto “0.”

FIG. 5 illustrates an exemplary shift register element, shift registerelement 25 of LFSR circuit 120 of FIG. 2. This shows the programmableelements and signals that provide the configurability of individualshift register element consistent with aspects of an embodiment of theinvention. As illustrated, configurable shift register circuit 25includes flip-flop 50 and muxes 51, 52, and 53, all coupled as shown.Muxes 51, 52, and 53 are controlled, respectively, by signalsCTRL_ADVANCE, CTRL_INIT, and CTRL_LOAD.

Circuit 25 outputs signal SCRM_NEXT at the output of mux 51 and signalLFSR_OUT at the output of flip-flop 50. LFSR_OUT is provided as a“SCRM_CUR” signal of the next shift register circuit in the chain (inthis example, shift register circuit 26, not separately shown in detail,but referenced in FIG. 2). In like fashion, the SCRM_CUR signalillustrated in FIG. 5 is provided to the B input of mux 51 and isprovided from an “LFSR_OUT” output signal of a prior shift registerelement circuit (in this example, shift register element circuit 24, notseparately shown in detail, but referenced in FIG. 2). Signal“SCRM_SEED” is provided for use when the LFSR circuit of which circuit25 is a part is to be utilized as an additive scrambler/descrambler aswill be further described below.

Use of control signals CTRL_ADVANCE, CTRL_INIT, and CTRL_LOAD to adaptcircuit 25 for operation as part of a particular type ofscrambler/descrambler is illustrated by the following table:

TABLE I CTRL_ADVANCE CTRL_INIT CTRL_LOAD Multiplicative B A ScramblerMultiplicative B A Descrambler Additive A then B B B then A ScramblerAdditive A then B A or B B then A DescramblerWith reference to FIG. 5 and the above table, to configure circuit 25 aspart of an LFSR multiplicative scrambler or descrambler circuit (in thisexample, circuit 120), signal CTRL_ADVANCE is set to select the B inputof mux 51 and signal CTRL_LOAD is set to select the A input of mux 53.In this manner data is passed through the LFSR circuit. Because amultiplicative scrambler scrambles (or descrambles) data without needingan initial seed separate from the data, neither the “SCRM_SEED” nor the“PRESET” input is utilized. In some cases it might be desired to skipcertain sets of incoming words (for example, if a particular protocolincludes clock compensation words for retiming purposes). In such cases,it might be desirable to temporality set the “CRTL_ADVANCE” signal toselect the “A” input so that the scrambler is temporarily bypassed andits state is held and then later reset the signal to select the “B”input when regular data resumes.

To configure circuit 25 as part of an LFSR additive scrambler circuit,the CTRL_ADVANCE signal is set to first select the A input of mux 51.This freezes the state of the circuit so that an initial seed can beloaded. In this example, the initial seed is from the PRESET signalprovided at the input of mux 52. This preset value may be provided as aset value in a configuration RAM element or other programmable element.However, in another embodiment, the initial value may be provided by anon-chip state machine and/or sent from another device with which the ICthat includes LFSR 120 (and circuit 25) will be communicating. In suchan alternative, signal CTRL_INIT may be set to select the A input of mux52 and the initial seed value will be provided by signal SCRM_SEED.Continuing with this example, for an additive scrambler, signalCTRL_LOAD is initially set to select the B input of mux 53 so that theoutput of mux 52 can provide the initial scrambling seed to be loadedinto flip-flop 50. However, after an initialization event is used toload flip-flop 50, then (on a subsequent clock cycle) signalsCTRL_ADVANCE and CTRL_LOAD are changed so that, respectively, the Binput of mux 51 and the A input of mux 53 are selected. This enables theoperational mode and data can now be loaded into shift register elementcircuit 25 and scrambled accordingly. If LFSR circuit 120 is to beutilized as an additive descrambler, then control signals are providedin a similar manner except that, in one example, signal CTRL_INIT is setto select the A input of mux 52. Thus the initial seed is provided bythe signal SCRM_SEED. In one example, SCRM_SEED is provided by anon-chip state machine. In another example, SCRM_SEED is extracted from aportion of the incoming data stream that is to be descrambled. In analternative embodiment, if the seed needed for descrambling is known inadvance, that seed may be provided as a programmed PRESET signal and theCTRL_INIT signal may be set to select the B input of mux 52 such thatPRESET provides the initial descrambling seed. Note that in someembodiments, the PRESET value is stored as a configuration elementsetting and might be different for different lanes.

Having a mux such as mux 53 in front of each flip-flop in LFSR circuitsin each lane also allows switching from a multiplicative scrambler to anadditive scrambler when LFSR circuits from multiple lanes are usedtogether (cascade mode). For an additive scrambler, the necessary seedcan be loaded into the shift register elements of an LFSR in one of thelanes by selecting the B input of muxes such as mux 53 during a loadsequence and then switched to select the A input of those muxes afterthe seed is loaded and that seed can then propagate throughout thescrambler portions in other lanes.

FIG. 6 illustrates scrambling lane circuitry 600 in according with analternative embodiment of the invention adapted for a 4-bit wide datalane. Although FIGS. 1, 2, 3, and 5 described embodiments of theinvention in the context of examples in which each data lane was assumedto handle data that is a single bit wide, in other examples a given lanemight have data that is more than one bit wide. FIGS. 6-7 illustratedexamples showing how configurable LFSR circuits can be built for eachlane to scramble data that is more than one bit wide per lane.

As illustrated in FIG. 6, LFSR circuit 600 includes 3-input muxes 621,622, 623, and 624; 2-input muxes 625, 626, 627, and 628; flip-flops 601,602, 603, and 604; input XOR circuits 611, 612, 613, and 614; andfeedback XOR circuits 615, 616, 617, and 618. LFSR circuit 600 receivesdata input signals DATA_IN[10], DATA_IN[11], DATA_IN[12], andDATA_IN[13] and scrambling input signals SCRM_IN[10], SCRM_IN[11],SCRM_IN[12], and SCRM_IN[13]. The outputs of flip-flops 604, 603, 602,and 601 provide, respectively, signals LFSR_OUT[10], LFSR_OUT[11],LFSR_OUT[12], and LFSR_OUT[13]. The outputs of XOR circuits 614, 613,612, and 611 provide, respectively, signals DATA_OUT[10], DATA_OUT[11],DATA_OUT[12], and DATA_OUT[13]. The outputs of muxes 624, 623, 622, and621 provide, respectively, signals SCRM_NEXT[10], SCRM_NEXT [11],SCRM_NEXT [12], and SCRM_NEXT [13].

LFSR circuit 600 is configurable to operate in either cascade ornon-cascade mode. If muxes 625, 626, 627, and 628 are configured toselect their B inputs, then LFSR circuit 600 operates in non-cascademode. In this mode, LFSR circuit 600 acts as a stand-alone scrambler forone lane of 4-bit wide data. However, if muxes 625, 626, 627, and 628are configured to select their A inputs, then LFSR circuit 600 operatesin cascade mode. In this mode, LFSR circuit 600 acts together withsimilar circuits in other lanes to scramble data across multiple lanes.Specifically, in cascade mode, muxes 625, 626, 627, and 628 select asinput, respectively signals SCRM_IN[10], SCRM_IN[11], SCRM_IN[12], andSCRM_IN[13]. These signals are the “SCRM_NEXT” signals from the outputsof 3-input muxes in a similar LFSR circuit in another lane (notseparately shown).

LFSR circuit 600 is also configurable to operate as a differentscrambler “type” in a similar manner to that described in the context ofFIG. 2. With reference to FIG. 6, and assuming the scrambler isconfigured in “non-cascade” mode as described above, if LFSR circuit 600is to be utilized as a multiplicative scrambler, then muxes 621, 622,623, and 624 are configured to select their B inputs. When LFSR circuit600 is to be utilized as a multiplicative descrambler, the muxes 621,622, 623, and 624 are configured to select their A inputs. When LFSRcircuit 600 is to be utilized as an additive scrambler or descrambler,then muxes 621, 622, 623, and 624 are configured to select their Cinputs.

FIG. 7 illustrates scrambling lane circuitry in accordance with analternative embodiment of the invention adapted for a 5-bit wide datalane. FIG. 6 and FIG. 7, taken together, show the structuralrequirements for scrambling lane circuitry that is more than one bitwide. As illustrated in FIG. 7, LFSR circuit 700 includes 3-input muxes721, 722, and 723; 2-input muxes 725, 726, 727, and 728; flip-flops 701,702, 703, and 704; input XOR circuits 711, 712, 713, 714 and 715; andfeedback XOR circuits 716, 717, 718, 719, and 720. LFSR circuit 700receives data input signals DATA_IN[10], DATA_IN[11], DATA_IN[12],DATA_IN[13] and DATA_IN[14] and scrambling input signals SCRM_IN[10],SCRM_IN[11], SCRM_IN[12], and SCRM_IN[13]. The outputs of flip-flops704, 703, 702, and 701 provide, respectively, signals LFSR_OUT[10],LFSR_OUT[11], LFSR_OUT[12], and LFSR_OUT[13]. The outputs of XORcircuits 715, 714, 713, 712, and 711 provide, respectively, signalsDATA_OUT[10], DATA_OUT[11], DATA_OUT[12], DATA_OUT[13], andDATA_OUT[14]. The outputs of muxes circuits 724, 723, 722, and 721provide, respectively, signals SCRM_NEXT[10], SCRM_NEXT [11], SCRM_NEXT[12], and SCRM_NEXT [13].

As described in the context of LFSR circuit 600 of FIG. 6, LFSR 700circuit 700 is configurable to operate in either cascade mode ornon-cascade mode by configuring muxes 725, 726, 727, and 728 to selecttheir A inputs (for cascade mode) or B inputs (for non-cascade mode).Also, in similar fashion to LFSR 600, LFSR 700 can be configure tooperate as a different type of scrambler (additive versusmultiplicative, scrambler versus descrambler) by configuring muxes 712,722, 723, and 724 as described for the comparable muxes in FIG. 6.

FIG. 7 illustrates how a wider data path can be accommodated within alane-based scrambler. Specifically, the number of flip-flops is the sameas in FIG. 6 (four flip-flops) and correlates to the highest exponent inthe scrambling polynomial. However, additional data inputs and outputsare accommodated with additional XOR circuits 715 and 720 as shown andthe wiring connections are modified accordingly as shown in FIG. 7.

In the examples of FIG. 6 and FIG. 7, the supported polynomial isx⁴+x+1, a fourth degree polynomial with three terms. However, a largernumber of terms in a scrambling polynomial could be supported inalternative embodiments by providing additional 3-input multiplexers(for example, an additional 3-input mux would be provided in front ofinput XOR circuit 715 if LSFR circuit 700 needed to implement apolynomial with four terms; and, as those skilled in the art wouldunderstand, to the extent such a polynomial also has a highest exponentof more than four, the number of flip-flops such as flip-flops 701-704and associated muxes 725-728 would be expanded accordingly). Followingthe pattern illustrated in FIGS.6 and 7,one skilled in the art couldprovide a configurable LFSR circuit for various data widths.

FIG. 8 illustrates lane reordering circuitry 800 coupled to provideconfigurability between least significant bit (“LSB”) and mostsignificant bit (“MSB”) ordering for any two or more bonded lanes (ofthe four illustrated lanes) in accordance with an aspect of anembodiment the invention. Specifically, circuitry 800 includes muxes 81,82, 83, 84, 85, 86, 87, and 88. The respective outputs of muxes 81, 82,83, and 84 are respectively coupled to respective scrambler circuitry600-0, 600-1, 600-2, and 600-3 via respective bit reversal circuitry900-0, 900-1, 900-2, and 900-3 in, respectively, LANE 0,LANE 1 ,LANE 2,and LANE 3. The scrambler circuitry in each lane is coupled throughrespective bit reversal circuitry as shown to muxes 85-88 as follows:scrambler 600-0 is coupled (via bit reversal circuitry 900-0) to “A”inputs of muxes 85, 86, 87, 88, and 89; scrambler 600-1 is coupled (viabit reversal circuitry 900-1) to “B” inputs of muxes 85, 86, 87, 88, and89; scrambler 600-2 is coupled (via bit reversal circuitry 900-2) to “C”inputs of muxes 85, 86, 87, 88, and 89; and scrambler 600-3 is coupled(via bit reversal circuitry 900-3) to “D” inputs of muxes 85, 86, 87,88, and 89. The A inputs of, respectively, muxes 81, 82, 83, and 84 arecoupled to receive input signal DATA_IN[0]. The B inputs of,respectively, muxes 81, 82, 83, and 84 are coupled to receive inputsignal DATA_IN[1]. The C inputs of, respectively, muxes 81, 82, 83, and84 are coupled to receive input signal DATA_IN[2]. The D inputs of,respectively, muxes 81, 82, 83, and 84 are coupled to receive inputsignal DATA_IN[3]. The output of, respectively, muxes 85, 86, 87, and 88are coupled to provide respective output signals DATA_OUT[0],DATA_OUT[1], DATA_OUT[2], and DATA_OUT[3].

The illustrated arrangement allows any two or more illustrated lanes tobe aggregated and to select between LSB and MSB ordering for theaggregated lanes. For example, if all four lanes are aggregatedtogether, then to use circuitry 800 with a protocol using LSB ordering,the circuitry would be configured as follows: muxes 81 and 85 areconfigured to select their A inputs; muxes 82 and 86 are configured toselect their B inputs; muxes 83 and 87 are configured to select their Cinputs; and muxes 84 and 88 are configured to select their D inputs. Touse circuitry 800 with a protocol requiring MSB ordering (with all fourlanes aggregated together), the circuitry would be configured asfollows: muxes 81 and 85 are configured to select their D inputs; muxes82 and 86 are configured to select their C inputs; muxes 83 and 87 areconfigured to select their B inputs; and muxes 84 and 88 are configuredto select their A inputs.

As another example, if just LANE 0, LANE 1, and LANE 2 were aggregatedtogether then, to use circuitry 800 with a protocol using LSB orderingfor those lanes, the circuitry would be configured as follows: muxes 81and 85 are configured to select their A inputs; muxes 82 and 86 areconfigured to select their B inputs; and muxes 83 and 87 are configuredto select their C inputs. To use circuitry 800 with a protocol using MSBordering for those lanes, the circuitry would be configured as follows:muxes 81 and 85 are configured to select their C inputs; muxes 82 and 86are configured to select their B inputs; muxes 83 and 87 are configuredto select their A inputs.

In some embodiments, A, B, C, and D inputs of each mux receive (and theoutputs of each mux transmit) data that is one bit wide. When each lanehandles data that is more than one bit wide, then additional bitordering circuitry 900 will be needed in each lane as further describedbelow in the context of FIG. 9.

It should be noted that if a particular application requires data pathwidth conversion (“gearboxing”) to change the width of incoming oroutgoing data, it may be necessary to reorder lanes after suchgearboxing on a transmit path and before such gearboxing on a receivepath and therefore gearboxing would occur between lane reordering andscrambling/descrambling. Thus, although not shown in FIG. 8 to avoidover complicating the drawings, in a particular implementation,gearboxing circuitry might be on a data path between lane reorderingcircuitry and bit reordering circuitry. For example, on a receivedatapath, such gearboxing circuitry would be after muxes 81-84 butbefore the bit reordering and descrambling circuitry of LANE 0 -LANE 3.On a transmit data path, such gearboxing circuitry would be after thescrambling circuitry (and after the bit reordering circuitry) of LANE 0-LANE 3 but before muxes 85-88.

FIG. 9 illustrates details of the bit reordering circuitry 900 of FIG. 8for a particular lane. Specifically, FIG. 9 illustrates bit reorderingcircuitry 900-1 coupled to LFSR 600-1 of FIG. 8 for a LANE 1. Circuitrysuch as circuitry 900-1 can be used to accomplish bit reversal when datais more than one bit wide per lane. Circuitry 900-1 includes muxes 91,92, 93, 94, 95, 96, 97, and 98. The outputs of muxes 91, 92, 93, and 94are respectively coupled to LFSR inputs, respectively, 901, 902, 903,and 904. The LFSR outputs 905, 906, 907, and 908 are respectivelycoupled to A inputs of, respectively, muxes 95, 96, 97, and 98 and to Binputs of, respectively, muxes 98, 97, 96, and 95. The A inputs of,respectively, muxes 91, 92, 93, and 94 are coupled to receive respectiveinput signals DATA_IN[10], DATA_IN[11], DATA_IN[12], and DATA_IN[13].The B inputs of, respectively, muxes 94, 93, 92, and 91 are also coupledto receive respective input signals DATA_IN[10], DATA_IN[11],DATA_IN[12], and DATA_IN[13]. The output of, respectively, muxes 95, 96,97, and 98 are coupled to provide respective output signalsDATA_OUT[10], DATA_OUT[11], DATA_OUT[12], and DATA_OUT[13].

If circuitry 900-1 is to be used with a protocol using LSB ordering,then muxes 91-98 are all configured to select their A inputs. Ifcircuitry 900 is to be used with a protocol using MSB ordering, thenmuxes 91-98 are all configured to select their B inputs. In a typicalapplication, the selected ordering (LSB or MSB) is done on the transmitside at the bit level and then distributed (“striped”) across lanes.Therefore, the settings for circuitry 800 in FIG. 8 and circuitry 900-1in FIG. 9 are typically related.

Scrambler 100 in FIG. 1 (or, scramblers in accordance with alternativeembodiments of the invention), may be provided as part of a transceiverin any IC that supports transceiver configuration. Such configurationmay be accomplished via data stored in programmable elements on the IC.Programmable elements may include dynamic or static RAM, flip-flops,electronically erasable programmable read-only memory (EEPROM) cells,flash, fuse, anti-fuse programmable connections, or other memoryelements. Transceiver configuration may also be accomplished via one ormore externally generated signals received by the IC during operation ofthe IC. Data represented by such signals may or may not be stored on theIC during operation of the IC. Transceiver configuration may also beaccomplished via mask programming during fabrication of the IC. Whilemask programming may have disadvantages relative to some of the fieldprogrammable options already listed, it may be useful in certain highvolume applications.

A specific example of an IC that supports transceiver configuration is aPLD. PLDs (also referred to as complex PLDs, programmable array logic,programmable logic arrays, field PLAs, erasable PLDs, electricallyerasable PLDs, logic cell arrays, field programmable gate arrays, or byother names) provide the advantages of fixed ICs with the flexibility ofcustom ICs. PLDs have configuration elements (i.e., programmableelements) that may be programmed or reprogrammed. Placing new data intothe configuration elements programs or reprograms the PLD's logicfunctions and associated routing pathways.

FIG. 10 illustrates an exemplary data processing system 1000 includingPLD 1001. PLD 1001 includes transceiver 1002 which includes a scrambler1001; scrambler 1001 is in accordance with an embodiment of theinvention. For ease of illustration, only a single transceiver is shown;however, a PLD such as PLD 1001 may include multiple transceivers suchas transceiver 1002.

Data processing system 1000 may include one or more of the followingadditional components: processor 1040, memory 1050, input/output (I/O)circuitry 1020, and peripheral devices 1030 and/or other components.These components are coupled together by system bus 1065 and arepopulated on circuit board 1060 which is contained in end-user system1070. A data processing system such as system 1000 may include a singleend-user system such as end-user system 1070 or may include a pluralityof systems working together as a data processing system.

System 1000 can be used in a wide variety of applications, such ascomputer networking, data networking, instrumentation, video processing,digital signal processing, or any other application where the advantageof using programmable or reprogrammable logic in system design isdesirable. PLD 1001 can be used to perform a variety of different logicfunctions. For example, PLD 1001 can be configured as a processor orcontroller that works in cooperation with processor 1040 (or, inalternative embodiments, a PLD might itself act as the sole systemprocessor). PLD 1001 may also be used as an arbiter for arbitratingaccess to shared resources in system 1000. In yet another example, PLD1001 can be configured as an interface between processor 1040 and one ofthe other components in system 1000. It should be noted that system 1000is only exemplary.

In one embodiment, system 1000 is a digital system. As used herein adigital system is not intended to be limited to a purely digital system,but also encompasses hybrid systems that include both digital and analogsubsystems.

Additional Embodiments

A first additional embodiment provides a scrambler configurable to beeither an additive or a multiplicative scrambler. In some variations ofthe first additional embodiment, the scrambler includes a data input, afirst selection circuit, an input XOR circuit, a plurality of shiftregister elements coupled together, and one or more feedback XORcircuits coupled to two or more of the plurality of shift registerelements; wherein: the data input is coupled to an input of the inputXOR circuit and to a first input of the first selection circuit; anoutput of the input XOR circuit is coupled to a second input of thefirst selection circuit; two of the shift register elements are coupledto inputs of the feedback XOR circuits which are in turn coupled to theinput XOR circuit and to a third input of the first selection circuitand further wherein: configuring the first selection circuit to selectits first input adapts the scrambler for operation as a multiplicativedescrambler; configuring the first selection circuit to select itssecond input adapts the scrambler for operation as a multiplicativescrambler; and configuring the first selection circuit to select itsthird input adapts the scrambler to operate as an additive scrambler ordescrambler. In some variations, the feedback XOR circuit includes aplurality of XOR circuits arranged in a tree, the feedback XOR circuittree being coupled to more than two of the shift register elements. Insome embodiments, a scrambling seed for an additive scrambler may bepre-programmed into configuration elements coupled to configurablecircuits for loading individual shift register elements. In otherembodiments, the scrambling seed may be provided through a data input asan initial part of a data stream. In some embodiments, selectioncircuits coupled to each shift register element in the configurable LFSRcircuit may be controlled to first load a scrambling seed and thenevolve data in an operational mode to provide scrambled data. In someembodiments, the scrambling seed value for a particular shift registerelement may be provided from a selected one of multiple input sources.

A second additional embodiment provides a scrambler configurable tosupport any one of a plurality of scrambling polynomials. Somevariations of the second additional embodiment include a plurality ofprogrammable taps coupled to outputs of shift register elements in anLFSR circuit. Outputs of the programmable taps are coupled to a feedbackXOR circuit tree. In one variation, the programmable taps comprise ANDgates with one input of the AND gate being coupled to an output of ashift register element and another input being coupled to aconfiguration element. In one variation, the number of individual XORcircuits in the XOR circuit tree and/or the number of programmable tapsis streamlined such that only the outputs of shift register elementswhose place in the LFSR circuit correspond to exponent values appearingin potentially supported scrambling polynomials are coupled to the XORcircuit tree. In one aspect, the configurable scrambler may beprogrammed for supporting a particular polynomial by programming “1” ineach configuration element that is coupled to an AND gate input for ANDgates coupling shift register elements whose place in the LFSR circuitcorrespond to exponent values of terms in the selected scramblingpolynomial for which the scrambler is to be configured.

A third additional embodiment includes a method for providing astreamlined multi-protocol LFSR circuit. The method includes providing anumber of shift register elements corresponding to the largest exponentvalue of any term in a scrambling polynomial to be supported; providingprogrammable taps coupling the output of each i^(th) shift registerelement—where “i” corresponds to an exponent value of a term in asupported scrambling polynomial—to a XOR circuit in a XOR circuit tree;providing a number of XOR circuits in the XOR circuit tree equal to oneless than the number of shift register elements with output coupled (viathe programmable taps) to the XOR circuit tree.

While the invention has been particularly described with respect to theillustrated embodiments, it will be appreciated that variousalterations, modifications and adaptations may be made based on thepresent disclosure, and are intended to be within the scope of theinvention. While the invention has been described in connection withwhat are presently considered to be the most practical and preferredembodiments, it is to be understood that the invention is not limited tothe disclosed embodiments but only by the following claims.

What is claimed is:
 1. A configurable scrambler in an integrated circuit(“IC”) for supporting a plurality of scrambling polynomial functions,the configurable scrambler comprising: a plurality of shift registerelements at least equal in number to a highest exponent value for a termwithin the plurality of scrambling polynomial functions; andprogrammable taps fewer in number than the plurality of shift registerelements, the programmable taps coupled between each output of a subsetof the plurality of shift register elements and a feedback XOR circuittree, wherein the programmable taps are configurable to select betweeneach of the plurality of scrambling polynomial functions, and whereinthe feedback XOR circuit tree includes a plurality of XOR circuits, theplurality of XOR circuits including at least a first XOR circuit and asecond XOR circuit, the first XOR circuit being coupled to receiverespective XOR inputs from respective different ones of the programmabletaps and the second XOR circuit being coupled at one of its XOR inputsto an XOR output of the first XOR circuit.
 2. The configurable scramblerof claim 1, wherein the programmable taps are coupled between outputs ofeach “i”th shift register element of the plurality of shift registerelements and the feedback XOR circuit tree, where “i” corresponds to avalue of a non-zero exponent for each term of the plurality ofscrambling polynomial functions.
 3. The configurable scrambler of claim2, wherein the feedback XOR circuit tree includes N XOR circuits, whereN +1 is equal in number to the plurality of shift register elements withoutputs coupled to programmable taps.
 4. The configurable scrambler ofclaim 1, wherein each of the programmable taps comprise a configurationelement coupled to an AND gate.
 5. The configurable scrambler of claim1, further comprising one or more selection circuits configurable toselect between a plurality of configuration options.
 6. The configurablescrambler of claim 5, wherein selecting one of the plurality ofconfiguration options includes one or more of selectively bypassing thescrambler, selecting between a cascade mode and an independent mode, andselecting a scrambler type.
 7. The configurable scrambler of claim 6,wherein selecting the scrambler type includes adapting the scrambler tooperate as one of an additive scrambler, an additive descrambler, amultiplicative scrambler and a multiplicative descrambler.
 8. Theconfigurable scrambler of claim 5, further comprising one or moreconfiguration elements configurable to control input to the one or moreselection circuits.
 9. The configurable scrambler of claim 8, whereinthe one or more configuration elements include at least one of a randomaccess element, a one-time programmable element, and on-chip processinglogic.
 10. The configurable scrambler of claim 1, further comprising anXOR circuit for performing an XOR operation between a result of thefeedback XOR circuit tree and input data.
 11. A programmable logicdevice comprising the configurable scrambler of claim
 1. 12. Aprogrammable logic device comprising the configurable scrambler of claim2.
 13. A data processing system comprising the programmable logic deviceof claim
 11. 14. A data processing system comprising the programmablelogic device of claim
 12. 15. A method of configuring a scrambler tosupport a selected scrambling polynomial function, the scramblerincluding a plurality of shift register elements at least equal innumber to a highest exponent value for a term within a plurality ofscrambling polynomial functions, and programmable taps fewer in numberthan the plurality of shift register elements, the programmable tapscoupled between each output of a subset of the plurality of shiftregister elements and a feedback XOR circuit tree, the feedback XORcircuit tree including a plurality of XOR circuits, the plurality of XORcircuits including at least a first XOR circuit and a second XORcircuit, the first XOR circuit being coupled to receive respective XORinputs from respective different ones of the programmable taps and thesecond XOR circuit being coupled at one of its XOR inputs to an XORoutput of the first XOR circuit, the method comprising: configuringprogrammable taps coupled between outputs of each “i”th shift registerelement of the plurality of shift register elements and the feedback XORcircuit tree to pass an output value of a corresponding shift registerelement to the feedback XOR circuit tree, wherein “i” corresponds to avalue of a non-zero exponent for a term of the selected scramblingpolynomial function; and configuring remaining programmable taps coupledbetween outputs of the plurality of shift register elements and thefeedback XOR circuit tree to output a logical value of “0”.
 16. Themethod of claim 15, further comprising: coupling the programmable tapsbetween outputs of each “i”th shift register element of the plurality ofshift register elements and a feedback XOR circuit tree, wherein “i”corresponds to a value of a non-zero exponent for a term of theplurality of scrambling polynomial functions.
 17. The method of claim15, further comprising providing one or more selection circuitsconfigurable to select between a plurality of configuration options. 18.The method of claim 17, further comprising selecting between theplurality of configuration options, wherein the plurality ofconfiguration options include selectively bypassing the scrambler,selecting between a cascade mode and an independent mode, and selectinga scrambler type.
 19. The method of claim 17, further comprisingproviding one or more configuration elements configurable to controlinput to the one or more selection circuits.
 20. The method of claim 15,further comprising providing a result of the feedback XOR circuit treeto an XOR circuit for performing an XOR operation between the result andinput data.
 21. A configurable scrambler in an integrated circuit (“IC”)for supporting a plurality of scrambling polynomial functions, theconfigurable scrambler comprising: a plurality of shift registerelements; and programmable taps fewer in number than the plurality ofshift register elements, the programmable taps coupled between eachoutput of a subset of the plurality of shift register elements and afeedback XOR circuit tree, wherein each shift register element of thesubset of the plurality of shift register elements corresponds to anon-zero exponent term within the plurality of scrambling polynomialfunctions, wherein the programmable taps are configurable to selectbetween each of the plurality of scrambling polynomial functions, andwherein the feedback XOR circuit tree includes a plurality of XORcircuits, the plurality of XOR circuits including at least a first XORcircuit and a second XOR circuit, the first XOR circuit being coupled toreceive respective XOR inputs from respective different ones of theprogrammable taps and the second XOR circuit being coupled at one of itsXOR inputs to an XOR output of the first XOR circuit.
 22. Theconfigurable scrambler of claim 21, wherein a quantity of the subset ofthe plurality of shift register elements is less than, or equal to, anumber of non-zero exponent terms within the plurality of scramblingpolynomial functions.
 23. The configurable scrambler of claim 21,wherein the feedback XOR circuit tree includes N XOR circuits, where N+1 is equal in number to the plurality of shift register elements. 24.The configurable scrambler of claim 21, wherein each of the programmabletaps comprise a configuration element coupled to an AND gate.
 25. Theconfigurable scrambler of claim 21, further comprising one or moreselection circuits configurable to select between a plurality ofconfiguration options.